A striking feature of Neisseria gonorrhoeae, the causative agent of gonorrhea, is its ability to vary expression of key surface components. The gonococcus expresses multiple antigenic types of Opa and pilin proteins, switching from one type to another during infection. Both Opa proteins and pili are adhesins for human cells. The long-term goal of this project is to understand the contributions of adhesins with different receptor binding specificities to gonococcal expression of Opa proteins with particular receptor binding specificities required for human infection? Preliminary results indicate that a mutant unable to express Opa proteins is non-infectious in the human challenge model. Restoring to the mutant the ability to express wild type or mutant Opa proteins with different receptor binding specificities and testing these strains in human challenge will determine if the requirement for Opa protein expression in infection depends solely upon its receptor binding function. 2) What determines the identify of the different antigenic types of pilin expression occurs during experimental infection, with the identify of the expressed variants differing between subjects. One hypothesis is that the organisms express adhesins "matching" the particular receptors on the surface of cells in different individuals. To test this hypothesis, subjects who have undergone one experimental infection will be reinfected, and adhesin variation during both the initial infection and the reinfection will be characterized. 3) What are the population dynamics of gonococcal infection within a human host? Is one organism of the appropriate phenotype sufficient to establish infection, and are there multiple stages of infection where selective bottlenecks occur? These questions will be addressed by tracking the appearance and disappearance of different variants during experimental infection and by characterizing organisms cultured from patients with natural gonorrhea. These studies will provide in vivo tests of the importance of interactions that have been identified solely on the basis of in vitro experiments.